1. Technical Field of the Invention
The present invention relates to a lossless compression-encoding device for data such as image data or audio data.
2. Description of the Related Art
Image data compression technologies can be mainly classified into lossy compression technologies and lossless compression technologies. Lossy compression technologies mostly use orthogonal transformation and have an advantage in that they obtain a high compression ratio although it is not possible to completely reconstruct original image data from compressed data.
On the other hand, lossless compression technologies have an advantage in that it is possible to completely reconstruct original image data from compressed data although a high compression ratio is not obtained. Therefore, lossless compression technologies are used for purposes such as storage or exchange of medical images. One lossless compression technology is described in Japanese Patent Application Publication No. Heisei 9-37271.
A lossless compression algorithm, that uses both differential coding and variable-length coding, is employed in JPEG-Lossless (JPEG-LS) of the Joint Photographic Experts Group (JPEG). This type of lossless compression algorithm cannot obtain sufficient prediction accuracy when the correlation between adjacent pixels of an image is weak. Therefore, the absolute value of a prediction error obtained through differential coding is generally large so that the total amount of data of a finally obtained variable-length code is large. Thus, it is not possible to achieve sufficient compression effects.